Thermal-overload protective arrangement for inductive devices

ABSTRACT

A thermal overload protective arrangement is provided for an inductive device having a core and coil assembly. The arrangement comprises a thermal overload protector which is supported on the inductive device in intimate heat exchange relation with the coil by means of a thermally conducting adhesive. A thermally insulating coating is then deposited on the surface of the thermal overload protector, remote from the coil, to prevent heat from the coil from being dissipated away before it reaches the thermal overload protector, thereby increasing the sensitivity of the device. The thermally insulating coating also serves to protect the thermal overload protector from an asphaltic potting composition which is poured, while hot, to enclose the entire inductive device.

United States Patent Earing 1 July4,1972

[54] THERMAL-OVERLOAD PROTECTIVE ARRANGEMENT FOR INDUCTIVE DEVICES [63] Continuation of Ser. No. 669,233, Sept. 20, 1967,

abandoned.

[52] US. Cl ..317/15 [51] Int. Cl. ..II02h 7/ 14 [58] Field of Search ..317/13, 15

[56] References Cited UNITED STATES PATENTS 3,119,913 1/1964 Benanderetal ..317/15X 3,173,059 3/1965 Stake ..3l7/15 3,219,856 11/1965 Dunwiddle et al..... ...317/13 X 3,275,774 9/1966 Miller ..317/15 3,386,063 5/1968 Mansfield, Jr. ...317/15 X 3,405,317 10/1968 Anderson ..317/15 3,435,291 3/1969 Paulus .l..3l7/l5 FOREIGN PATENTS OR APPLICATIONS 350,845 6/1931 Great Britain ..317/l5 383,197 11/1932 Great Britain ..317/15 Primary Examiner-William M. Shoop, Jr. Attorney-John M. Stoudt, Ralph E. Krisher, .Ir., Frank L. Neuhauser, Oscar B. Waddell, Radford M. Reams and Joseph B. Forman [5 7] ABSTRACT A thermal overload protective arrangement is provided for an inductive device having a core and coil assembly. The arrangement comprises a thermal overload protector which is supported on the inductive device in intimate heat exchange relation with the coil by means of a thermally conducting adhesive. A thermally insulating coating is then deposited on the surface of the thermal overload protector, remote from the coil, to prevent heat from the coil from being dissipated away before it reaches the thermal overload protector, thereby increasing the sensitivity of the device. The thermally insulating coating also serves to protect the thermal overload protector from an asphaltic potting composition which is poured, while hot, to enclose the entire inductive device.

18 Claims, 5 Drawing Figures P'A'TE'N'TEDJUL 4 m2 3,675,081

FIGI 25 INVENTOR MASON H. EARING BY W ATTORNEY BACKGROUND OF INVENTION The present invention relates to a thermally sensitive arrangement and, more particularly, to a thermal overload protective arrangement for inductive devices.

In many particular applications there is need for a thennal overload protective means for an inductive device such as, for example, a transformer used as part of the ballast of a fluorescent lamp. Such protection is necessary to prevent burning of the windings comprising the coil assembly due to an abnormal high current therethrough, and to prevent leakage of the potting or filling compound which encloses the transformer upon heating thereof dueto an abnormally high temperature within the ballast.

Many different types of thermal overload protectors are known in the art and are utilized in conjunction with such inductive devices. Often, a bimetal switch or-a fuse structure is provided to interrupt current through the inductive device, as where the bimetal switch opens or the fuse opens to interrupt current therethrough. Sometimes, a short circuit is effected across the inductive device so that current is caused to pass through the short circuit and, thereby protect the inductive device from adverse effects due to the high current present.

A most desirable characteristic of such a thermal overload protector is that it be extremely sensitive to even minor variations in heat energy emanating from the coil so that there will be no lag between the time the coil reaches a critical high temperature and the time at which the thermal overload protector reaches its critical operating temperature. It is most important, then, to have a good thermal or heat exchange relation between the coil and the thermal overload protector, since placement of the protector too distant from the coil and the resultant dissipation of heat could result in an undesirable lag time which might cause permanent damage to the inductive device.

, In an ordinary transformer wherein such a thermal overload protector is utilized, there is provided a core, normally iron or steel, comprising a series of stacked laminations suitably fastened to effect an integral structure. A suitable winding or coil is provided comprising layers of wire and foil which are wound over each other and then placed over upstanding portions of the core. Suitable output leads are connected to the coil and provide connecting means to a current source or load for the inductive device. As is well known in the art, the complete core and'coil assembly is generally placed in a hightemperature bath of varnish, shellac, or the like for impregnation therewith. Such impregnation reduces noise which normally exists because of vibrations and imparts a generally rigidity to the structure which is highly desirable.

It is normally at this stage of the construction of the completed inductor device that a thermal overload protector is placed in heat exchange relationship with the coil. Generally, mechanical means are utilized to support the thermal overload protector and'to position'it as close to the coil as is possible. Unfortunately, in practice, it has been found that the thermal overload protector is generally not in sufficiently intimate heat exchange relation with the coil so that an undesirable lag time results. This also can be attributed to heat dissipation or radiation away from the coil before it reaches the thermal overload protector in that no provision is made for containing the heat from the coil in the prior art arrangements. Furthermore, it is conventional in the art to pour a potting or filling compound over the inductive device to encapsulate it. Such a potting composition is normally asphaltic and must be poured hot, normally at a temperature of approximately 200 C. Because the potting composition directly contacts the thermal overload protector, there is a possibility that the heat generated may trigger the thermal overload protector prematurely. As a result, it may be necessary to use a thermal overload protector having too high a critical temperature, that is, one which will not be triggered by the potting composition but which may be too high to adequately protect the coil.

SUMMARY OF INVENTION It is therefore an object of the present invention to provide a thermal overload protective arrangement for an inductive device which is highly sensitive to temperature variations in the current carrying coil of the device.

It is another object of this invention to provide a thermal overload protective arrangement wherein dissipation of heat from the current carrying coil before it reaches the thermal overload protector is substantially minimized.

It is a further object of this invention to provide a thermal overload protective arrangement wherein the thermal overload protector is protected from prematurely operating when a hot potting or filling compound is poured to enclose the inductive device.

In accordance with my invention, in one form thereof, I provide a thermal overload protective arrangement for an inductive device including a core and a current-carrying coil. A thermal overload protector is supported on the coil in intimate heat exchange relation therewith by means of a thermally conducting adhesive. The surface of the thermal overload protector not in contact with the coil is coated with a thermally insulating material to prevent heat from being conducted away from the thermal overload protector. The thermal overload protector is connected to suitable leads brought out from the winding.

A particularly advantageous feature of the present invention is that the thermal overload protector is in good heat exchange relation with the current-carrying coil, thereby allowing it to be highly sensitive to heat variations therein. Another feature is that the thermally insulating material serves to substantially reduce the amount of heat which is dissipated before it reaches the thermal overload protector. Furthermore, the thermally insulating material serves to prevent the thermal overload protector from prematurely operating when the potting composition is poured to enclose the inductive device.

DETAILED DESCRIPTION Other objects and advantages of my invention may better be understood by reference to the following detailed description when taken in connection with the accompanying drawings in which:

FIG. 1 is a partially cut-away plan view of a transformer using the thermal overload protective arrangement of the present invention;

FIG. 2 is a sectional view of the transformer taken along line 2-2 of FIG. 1;

FIG. 3 is a plan view, partially cut away to show details, of the fuse shown in FIGS. 1 and 2;

FIG. 4 is a sectional view, similar to FIG. 2, of a modification of the present invention; and

FIG. 5 is a plan view, partially cut away to show details, of the fuse shown in FIG. 4.

Referring now to FIGS. 1 and 2 there is shown a form of the present invention wherein a transformer 1 is positioned within an outer housing 3. The transformer 1 includes a core 5 com prising a series of stacked laminations 7 of iron, steel or other magnetic alloys. The laminations are fastened together by suitable fastening means (not shown) to form an integral unit. The core 5 comprises a pair of outer leg portions 9 and 11 and a central leg portion 13 defining apertures 15. While the core 5 is shown in FIG. 1 having the particular preferred configuration, it is to be understood that no particular core configuration, or configuration of individual laminations, is necessary and that the configurations may vary as is necessary for the desired application. It is to be also understood that such a core member may be used with any number of windings, as required for the particular application desired.

Each aperture 15 of the core member is adapted to receive a winding or coil in FIG. 1. Each coil comprises a generally cylindrical member having a central aperture therethrough. The coil is formed by means of winding a wire or foil sheet around itself until a sufficiently requisite number of layers are produced and a coil of a desired thickness is produced. One coil constitutes the primary or current-carrying coil 19 and has leads 21 which may be connected to a power source. The other coil constitutes the secondary coil 23 and includes leads 25 which may be connected to a suitable lead. The power source and the transformer load are not shown in FIG. 1, and may constitute conventional devices, as are known to those skilled in the art.

A thermal overload protector is provided to sense the temperature of the current-carrying primary coil. The thermal overload protector may comprise a fuse 27, such as shown in FIG. 3, or any suitable heat-sensitive, circuit-opening device, as is well known to those skilled in the art. As is shown in FIG. 3, the fuse 27 includes a cylindrical housing 29 comprising a glass envelope 31 and a pair of metallic end caps 33 and 35. A pair of leads 37 and 39 are provided for the fuse 27, one lead extending from each end of the housing. A spring 36 is electrically connected at one of its ends to the housing 29 at end cap 35 and is connected at its other end to a heat-sensitive element such as a metal alloy piece 41 having a predetermined melting temperature. A contact lead 43 is connected at one end to a portion ofthe alloy piece 41 separated from the spring connection, and is connected at its other end to end cap 33. When the predetermined temperature of the metal alloy piece 41 is reached, the alloy piece melts, thereby breaking the electrical connection between the spring 36 and the contact lead 43. The terminal leads 37 and 39 from the fuse 27 are connected to leads 47 and 49 brought up from the primary coil 19 so as to place the fuse 27 in series with the primary coil 19. While the fuse is connected in series with the coil, the internal current through the fuse is negligible, the primary heat source being the heat emanating from the coil. As seen in FIG. 2, the fuse is lodged in the crevice formed by the core 5 and the primary coil 19.

In accordance with the present invention, to maintain the fuse 27 in intimate heat exchange relation with the primary coil 19, a rapid-setting adhesive or cement 51, such as that disclosed in my copending application, Ser. No. 669,252, filed Sept. 20, 1967, assigned to the same assignee as the present invention, is provided to secure the fuse 27 to the coil 19 and core 5. A filler material such as tabular alumina is added to the cement before mixing at high concentration to make the cement highly thermally conductive. It is understood, however, that other thermally conductive cements, as known to those skilled in the art, may be used. It is also to be understood that the fuse 27 may be supported completely on the coil 19, if desired, wherein the fuse 27 adheres to the coil 19 by means of the thermally conducting cement.

If desired, the ends of the terminal leads 47 and 49 from the primary coil 19 may be positioned at predetermined points to contact terminals 53 and 55, respectively, on the top lamination of the core 5. Terminal 55 is retained by end pad 58. This serves to predetermine the position of the fuse 27 with respect to the primary coil 19 so as to assure intimate heat exchange relation therewith. A further advantage in such placement, as is shown in FIG. 1, is that the leads 37 and 39 from the fuse 27 are not drastically bent, crimped or twisted during assembly. This reduces the residual stress in the leads to the fuse and further improves the reproducibility of its functioning temperature.

In many applications, the transformer assembly is impregnated with a varnish or shellac to diminish the noise characteristic of such devices. The impregnation is usually effected by submerging the entire core and coil assembly in a suitable bath, as is well known in the art. It has been found that the present adhesive composition exhibits exceptional adhering capabilities with respect to this type of surface.

The transformer l is usually suspended in' the housing 3 by means of a potting or filling composition 59 comprising a generally asphaltic material which is poured into the casing during assembly. Because the potting compound 59 is poured while hot, generally in the nature of 200 C., there is a possibility that the fuse 27 will prematurely trigger because the criticaltemperaturethereof is exceeded by the heat from the potting compound. In order to prevent such an occurrence and, generally to protect the fuse 27 from damage by the potting compound 59, the fuse 27 may be wrapped or otherwise covered with an insulating member or material. In a preferred form of the invention, such as that shown in FIGS. 1 and 2, a thermally insulating coating 61 may be provided to cover the surface of the fuse not contacted by the adhesive 51. Materials for use for such a purpose are well known, and may comprise a urethane or epoxy foam, for example. The thermal insulating coating 61 also has the highly advantageous feature of preventing heat from being dissipated, radiated or conducted away from the fuse 27 to the housing 3 while not interfering with the heat conducting path from the coil 19. This heat-retention characteristic serves to increase the sensitivity of the fuse 27 to minor variations in the temperature of the primary coil 19. Furthermore, this method of attachment serves as an encapsulation method to provide a protective barrier between the fuse and the asphalt fill 61.

In another embodiment of the present invention, as shown in FIG. 4, the thermal overload protector 63 is of the type described in US. Pat. No. 3,20l,646, issued to W. C. Mansfield, Jr., and assigned to the same assignee as the present invention. Protector 63 includes a wax body 65 and a pair of lead members 69 and 71 which are bridged by a fusible link 73. The lead members extend to be connected with suitable winding portions or leads brought up from the primary coil 19 so as to place the protector 63 in series with the primary coil 19. Again the series connection has little effect on the operation of the fuse, the heat emanating from the coil being the main factor. The wax body 65 is formed to surround the link 73. The link 73 and the wax body 65 are so fused to the lead members that, upon reaching a predetermined critical temperature, the link 73 is freed and may drop from its normal position, thereby opening the circuit. The wax body 65 melts when the critical temperature of link 73 is attained so that link 73 may freely drop to open the circuit. The wax body 65 also serves to prevent the protector 63 from triggering prematurely when the potting composition 61 is added. If desired, a preferred form of the invention, shown in FIG. 4, includes the epoxy foam coating 61 which may also be applied to further protect the protector 63 and to prevent heat from being radiated, dissipated or conducted away therefrom.

I have thus described a thermal overload protective arrangement wherein a thermal overload protector is supported in intimate heat exchange relation with the primary windin g of an inductive device by means of a thermally conducting adhesive. A thermally insulated coating is also provided to cover surface portions of the thermal overload protector not contacting the adhesive so as to effectively encapsulate the thermal overload protector and prevent premature triggering thereof when hot potting compound is introduced into the casing. Furthermore, the coating serves to prevent the dissipation, radiation or conduction of heat away from the overload protector to the casing, thereby increasing the sensitivity of the device to temperature variations in the primary coil.

While the present invention has been described with specificity, it is the aim of the appended claims to cover all such equivalent variations as come within the true spirit and scope of the invention.

What Iclaim is new and desire to secure by Letters Patent of the United States is:

1. In an inductive device including a core and a current-carrying coil, a thermal overload protective arrangement therefor, comprising:

a. a thermal overload protector operative at a predetermined critical temperature; 1

b. thermally conductive cement means adhering to said inductive device and to said thermal overload protector to support said thermal overload protector in intimate heat exchange relation with said coil, wherein heat is transferred from said coil to said thermal overload protector through said thermally conductive cement, and

c. means electrically connecting said coil in series with said thermal overload protector.

2. The device as recited in claim 1 wherein said thermal overload protector includes an operating portion and a meltable wax body covering the operating portion thereof.

3. The device as recited in claim 2 wherein said wax body is adapted to melt near the operating temperature of the thermal overload protector.

4. The device as recited in claim 1 wherein said thermal overload protector is supported by said core and said coil.

5. The device as recited in claim 1 wherein said thermal overload protector is supported entirely on said coil.

6. The device as recited in claim 1 wherein said thermal overload protector is provided with a pair of leads, one lead at each end thereof, and said coil includes a pair of leads connected to the leads of said thermal overload protector so that said thermal overload protector is in series with said coil.

7. The device as recited in claim 6 wherein terminal means are located at predetermined positions on said core, and said leads of said thermal overload protector are connected to said terminal means.

-8. In an inductive device including a core and a current-carrying coil, a thermal overload protective arrangement therefor, comprising:

a. a thermal overload protector operative at a predetermined critical temperature;

b. thermally conductive adhesive means for supporting said thermal overload protector in intimate heat exchange relation with said coil,

c. a thermally insulating material coating the surface of said thermal overload protector which is not contacting said thermally conductive adhesive, and

d. means electrically connecting said coil in series with said thermal overload protector.

9. The device as recited in claim 8 wherein said thermally insulating material comprises a urethane foam.

10. In an inductive device including a core and a currentcarrying coil, a thermal overload protective arrangement therefor, comprising:

a. a thermal overload protector operative at a predetermined critical temperature, including an operating portion and a meltable wax body covering the operating portion thereof,

b. thermally conductive adhesive means for supporting said thermal overload protector in intimate heat exchange relation with said coil,

c. a thermally insulating material coating the surface of said thermal overload protector which is not contacting said thermally conductive adhesive, and

d. means electrically connecting said coil in series with said thermal overload protector.

11. In an inductive device including a core and at least one coil, a thermal overload protective arrangement therefor, comprising:

a. a thermal overload protector operative at a predeter' mined temperature;

b. thermally conductive cement means adhering to said inductive device and to said thennal overload protector to support said thermal overload protector in intimate heat exchange relation with said inductive device, wherein heat is transferred from said inductive device to said thermal overload protector through said thermally conductive cement, and

0. means electrically connecting at least one of said coils in series with said thermal overload protector.

12. The device as recited in claim 11 wherein the surface of said thermal overload protector which is not contacting said thermally conductive cement means rs covered with a thermally insulating material.

13. The device as recited in claim 12 wherein said thermally insulating material comprises a urethane foam.

14. The device as recited in claim 11 wherein said thermal overload protector includes an operating portion and a meltable wax body covering the operation portion thereof.

15. The device as recited in claim 14 wherein said wax body is adapted to melt near the operating temperature of the thermal overload protector.

16. The device as recited in claim 14 wherein the surface of said thermal overload protector which is not contacting said thermally conducting cement means is covered with a thermally insulating material.

17. The device as recited in claim 11 wherein said thermally conductive cement means adheres to both said core and one of said coils, and to said thermal overload protector, such that said thermal overload protector is supported by both said core and one of said coils, wherein heat is transferred from both said core and one of said coils to said thermal overload protector through said thermally conductive cement.

18. The device as recited in claim 11 wherein said thermal overload protector is supported entirely on one of said coils.

i k a: 

1. In an inductive device including a core and a currentcarrying coil, a thermal overload protective arrangement therefor, comprising: a. a thermal overload protector operative at a predetermined critical temperature; b. thermally conductive cement means adhering to said inductive device and to said thermal overload protector to support said thermal overload protector in intimate heat exchange relation with said coil, wherein heat is transferred from said coil to said thermal overload protector through said thermally conductive cement, and c. means electrically connecting said coil in series with said thermal overload protector.
 2. The device as recited in claim 1 wherein said thermal overload protector includes an operating portion and a meltable wax body covering the operating portion thereof.
 3. The device as recited in claim 2 wherein said wax body is adapted to melt near the operating temperature of the thermal overload protector.
 4. The device as recited in claim 1 wherein said thermal overload protector is supported by said core and said coil.
 5. The device as recited in claim 1 wherein said thermal overload protector is supported entirely on said coil.
 6. The device as recited in claim 1 wherein said thermal overload protector is provided with a pair of leads, one lead at each end thereof, and said coil includes a pair of leads connected to the leads of said thermal overload protector so that said thermal overload protector is in series with said coil.
 7. The device as recited in claim 6 wherein terminal means are located at predetermined positions on said core, and said leads of said thermal overload protector are connected to said terminal means.
 8. In an inductive device including a core and a current-carrying coil, a thermal overload protective arrangement therefor, comprising: a. a thermal overload protector operative at a predetermined critical temperature; b. thermally conductive adhesive means for supporting said thermal overload protector in intimate heat exchange relation with said coil, c. a thermally insulating material coating the surface of said thermAl overload protector which is not contacting said thermally conductive adhesive, and d. means electrically connecting said coil in series with said thermal overload protector.
 9. The device as recited in claim 8 wherein said thermally insulating material comprises a urethane foam.
 10. In an inductive device including a core and a current-carrying coil, a thermal overload protective arrangement therefor, comprising: a. a thermal overload protector operative at a predetermined critical temperature, including an operating portion and a meltable wax body covering the operating portion thereof, b. thermally conductive adhesive means for supporting said thermal overload protector in intimate heat exchange relation with said coil, c. a thermally insulating material coating the surface of said thermal overload protector which is not contacting said thermally conductive adhesive, and d. means electrically connecting said coil in series with said thermal overload protector.
 11. In an inductive device including a core and at least one coil, a thermal overload protective arrangement therefor, comprising: a. a thermal overload protector operative at a predetermined temperature; b. thermally conductive cement means adhering to said inductive device and to said thermal overload protector to support said thermal overload protector in intimate heat exchange relation with said inductive device, wherein heat is transferred from said inductive device to said thermal overload protector through said thermally conductive cement, and c. means electrically connecting at least one of said coils in series with said thermal overload protector.
 12. The device as recited in claim 11 wherein the surface of said thermal overload protector which is not contacting said thermally conductive cement means is covered with a thermally insulating material.
 13. The device as recited in claim 12 wherein said thermally insulating material comprises a urethane foam.
 14. The device as recited in claim 11 wherein said thermal overload protector includes an operating portion and a meltable wax body covering the operation portion thereof.
 15. The device as recited in claim 14 wherein said wax body is adapted to melt near the operating temperature of the thermal overload protector.
 16. The device as recited in claim 14 wherein the surface of said thermal overload protector which is not contacting said thermally conducting cement means is covered with a thermally insulating material.
 17. The device as recited in claim 11 wherein said thermally conductive cement means adheres to both said core and one of said coils, and to said thermal overload protector, such that said thermal overload protector is supported by both said core and one of said coils, wherein heat is transferred from both said core and one of said coils to said thermal overload protector through said thermally conductive cement.
 18. The device as recited in claim 11 wherein said thermal overload protector is supported entirely on one of said coils. 